toolchain/ghs: greenhills compiler does not recognize .syntax unified

The GreenHills Compiler uses Unified Assembler Language by default and
it does not recognize .syntax unified instruction. Therefore, remove
them if GHS compiler is used.

Co-authored-by: Chengdong Wang <wangchengdong@lixiang.com>
Signed-off-by: xiezhanpeng3 <xiezhanpeng3@lixiang.com>

arch/arm/src/armv8-r/arm_fpuconfig.S

@@ -50,7 +50,9 @@
  ****************************************************************************/
 
 	.text
+#ifndef __ghs__
 	.syntax	unified
+#endif
 	.arm
 
 /****************************************************************************

arch/arm/src/armv8-r/arm_head.S

@@ -129,7 +129,9 @@
  */
 
 	.text
+#ifndef __ghs__
 	.syntax	unified
+#endif
 	.arm
 	.global	__start
 #ifdef __ghs__

arch/arm/src/armv8-r/arm_saveusercontext.S

@@ -30,7 +30,9 @@
 	.file	"arm_saveusercontext.S"
 
 	.text
+#ifndef __ghs__
 	.syntax	unified
+#endif
 	.arm
 
 /****************************************************************************

arch/arm/src/armv8-r/arm_vectors.S

@@ -137,7 +137,9 @@
  ****************************************************************************/
 
 	.text
+#ifndef __ghs__
 	.syntax	unified
+#endif
 	.arm
 
 /****************************************************************************

arch/arm/src/armv8-r/arm_vectortab.S

@@ -40,7 +40,9 @@
 	.eabi_attribute Tag_ABI_align_preserved, 1
 #endif
 
+#ifndef __ghs__
 	.syntax	unified
+#endif
 	.arm
 
 	.globl		_sys_vector_start

arch/arm/src/common/gnu/arm_fetchadd.S

@@ -26,7 +26,9 @@
 
 #include <nuttx/config.h>
 
+#ifndef __ghs__
 	.syntax		unified
+#endif
 	.file		"arm_fetchadd.S"
 
 /****************************************************************************